This invention relates to an optical scanning device for scanning an optical record carrier, such as an optical disk, including at least one information layer. The device includes one or more radiation sources for generating one or more radiation beams of one or more different wavelengths. In particular, but not exclusively, the invention relates to a device in which a radiation source is used during an optical record carrier write process which requires higher beam power at the spot on which the beam is focussed on the optical record carrier than an optical record carrier read process.
In the case of a disk format such as CD-R (Compact Diskxe2x80x94Recordable), the disks exhibit relatively high reflectance. For these, and other, formats, to increase the speed of writing data to a disk tends to require an increase in optical power level for recording purposes. At the same time, there is a desire to achieve a low-cost solution allowing the efficient manufacture of the device and its components in high volumes. When manufacturing components in high volumes, a key factor is the stability and reproducibility of the manufacturing process. If a process is unstable or difficult to reproduce, it has a significant effect on the final cost.
EP-A-1001414 describes an optical pickup capable of recording or reproducing information on or from compact disk formats, as well as reading from DVD disk formats. A single main optical path is used for both of the different wavelength beams, and respective beam splitters are used to couple the radiation beams into the main optical path. A monitoring device capable of detecting the output of light emitted from the fist and second optical sources is placed behind one of the beam splitters, outside the main optical path.
In accordance with the present invention there is provided an optical scanning device for scanning an optical record carrier, said device comprising a radiation source for emitting an incident radiation beam comprising a predetermined wavelength at a substantially linear polarisation, a reflective element located in the path of the incident radiation beam from the radiation source to the location of said optical record carrier and arranged to pass a first proportion, of the incident beam intensity which is incident upon the reflective element, towards the optical record carrier, a polarisation-altering element located in the path of the radiation beam from the reflective element to the location of said optical record carrier, and a detector for detecting a radiation beam reflected from said optical record carrier and passing along a path including the polarisation-altering element and the reflective element, the reflective element being arranged to pass a second proportion, of the reflected beam intensity incident upon the reflective element, towards the detector, the polarisation-altering element converting a linear polarisation of said incident beam into a substantially circular polarisation such that the incident and reflected beam have a different polarisation before and after passing through said polarisation-altering element respectively, characterised in that said reflective element is a partially polarising reflective element arranged such that said first proportion of intensity is greater, as a proportion, than said second proportion of intensity.
By use of a partially polarising reflective element in which the proportion of intensity passed for the polarisation of the incident beam is greater than the proportion of intensity passed for the polarisation of the beam reflected by the record carrier, a relatively high optical signal power at the optical record carrier can be achieved for recording purposes whilst allowing the reflective element to be manufactured in a relatively efficient manner. For the sake of clarity, the term xe2x80x9cproportionxe2x80x9d is intended to be understood as a fraction, namely the xe2x80x9cproportion of intensity passedxe2x80x9d is the intensity of the beam following the component divided by the incident of the beam before reaching the component, each intensity being taken at the centre of the beam. Using the invention, a relatively high level of transmission of the reflected beam towards the detector is also achievable, such that a desired data readout performance is achieved. A specified angular tolerance exhibited by the reflective element can also be achieved in a similar manner. Furthermore, the invention provides a reduced sensitivity to birefringence of the disk, which can in some known arrangements significantly reduce signal levels.
In one embodiment of the invention the device further comprises a sensor for sensing radiation from the incident beam transmitted through said reflective element during an optical record carrier write process, allowing the intensity at, or close to, the centre of the incident beam emitted by the radiation source to be monitored. Alternatively, the intensity of the incident beam may be monitored at the edge of the beam for example prior to its incidence on the reflective element. However, this is not preferred due to a significantly larger spread in signal levels.
The device may further comprise a second radiation source for emitting a second radiation beam comprising a different predetermined wavelength, and a second reflective element for reflecting said second radiation beam towards the location of said optical record carrier.
This arrangement further allows the invention to be implemented in a dual-wavelength light path device. In such a device, improved performance could be achieved using a perfect (or practically near-perfect) double polarising coating operating at the two different wavelengths. However, such a double polarising coating is very difficult to produce, unless intended use with parallel beams, in which case the optical pickup becomes more complex, bulky and expensive. By use of the present invention such problems may be mitigated. Thus, the first and/or second wavelength beams are preferably non-parallel when passing through the reflective element.
Further features and advantages of the present invention will become apparent from the following description of preferred embodiments of the invention.
It is noted that beam splitters having partially polarising reflection characteristics are known in the field of magneto-optic scanning devices. Such a device is described in U.S. Pat. No. 5,467,336. However, such arrangements are use for a different purpose and do not include a polarisation-altering element, such as the quarter wave plate used in embodiments of the present invention, whereby the polarisation of incident light is altered on reflection from the optical record carrier.